When is my next period?
Enter the first day of your last period and your usual cycle length to see when your next periods should start, and when you are most likely to ovulate.
- Next period starts
- October 13, 2026
With a 28-day cycle, your next period should start on October 13, 2026.
- Next period ends
- October 17, 2026
- Fertile window starts
- September 24, 2026
- Estimated ovulation
- September 29, 2026
- Period after that
- November 10, 2026
- Third period
- December 8, 2026
Answer for the example date Tuesday, September 29, 2026. It changes to today's date when the page loads.
Next period starts: October 13, 2026. With a 28-day cycle, your next period should start on October 13, 2026.
What is coming up in my cycle?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
Predicts the next three periods from the first day of the last period and the usual cycle length, with the likely ovulation day and fertile window.
Example with the default inputs (First day of your last period September 15, 2026, Cycle length (days) 28, Period length (days) 5, Today’s date September 29, 2026) on the example date Tuesday, September 29, 2026: With a 28-day cycle, your next period should start on October 13, 2026.
Method: Next period = the first of last period + k × cycle length (k = 1, 2, 3, …) on or after today; ovulation = next period − 14 days; fertile window = ovulation − 5 days to ovulation; later periods add the cycle length again.
- Every cycle has the same length as the one you enter. Real cycles vary from month to month.
- If your last period was more than one cycle ago, the dates continue the same rhythm, so the next period shown is never before today.
- Ovulation happens 14 days before the next period (the luteal phase), the average ACOG gives; in real cycles it varies.
- The fertile window is the 6 days that end on the ovulation day (Wilcox and others, 1995).
- The dates are estimates for information. They are not a reliable way to prevent pregnancy and are not medical advice.
Worked examples
Each example is checked against the calculator on every build.
- First day of your last period 2026-09-01, Cycle length (days) 28, Period length (days) 5 gives Next period starts 2026-09-29, Next period ends 2026-10-03, Fertile window starts 2026-09-10, Estimated ovulation 2026-09-15, Period after that 2026-10-27, Third period 2026-11-24.Source: hand calculation in content.mdx: ACOG (28-day cycle, ovulation about 14 days before the next period); Wilcox and others 1995 (6-day fertile window ending on ovulation)
- First day of your last period 2026-12-20, Cycle length (days) 32, Period length (days) 4 gives Next period starts 2027-01-21, Next period ends 2027-01-24, Estimated ovulation 2027-01-07, Fertile window starts 2027-01-02.Source: hand calculation in content.mdx: a 32-day cycle across the new year
- First day of your last period 2028-02-10, Cycle length (days) 21, Period length (days) 7 gives Next period starts 2028-03-02, Next period ends 2028-03-08, Estimated ovulation 2028-02-17, Fertile window starts 2028-02-12.Source: hand calculation in content.mdx: a 21-day cycle through February 29, 2028 (a leap year)
- First day of your last period 2026-06-01, Cycle length (days) 28, Period length (days) 5, Today’s date 2026-09-28 gives Next period starts 2026-10-19, Next period ends 2026-10-23, Fertile window starts 2026-09-30, Estimated ovulation 2026-10-05, Period after that 2026-11-16, Third period 2026-12-14.Source: hand calculation in content.mdx: 119 days since June 1; 119 ÷ 28 = 4.25, so k = 5 and June 1 + 140 days = October 19
- First day of your last period 2026-08-31, Cycle length (days) 28, Period length (days) 5, Today’s date 2026-09-28 gives Next period starts 2026-09-28, Period after that 2026-10-26.Source: hand calculation in content.mdx: 28 days since August 31, so k = 1 and the next period starts today
How the dates are worked out
All dates are calendar days. d is the first day of your last period, c your cycle length in days, p your period length in days, and t today's date (you can change it under "Count from another date").
- Next period starts on the first date d + k × c (k = 1, 2, 3, …) that is on or after t. In days: k = the larger of 1 and ⌈(t − d) ÷ c⌉. If your last period started less than one cycle ago, k = 1 and the next period is d + c. If it started longer ago, the dates continue the same rhythm, so the next period shown is never before today. A predicted day that falls on today counts as the next period. If today's date is left empty, k = 1.
- Next period ends on (d + kc) + (p − 1) days: a 5-day period that starts on the 29th ends on the 3rd of the next month when the month has 30 days.
- Estimated ovulation is 14 days before the next period: (d + kc) − 14.
- Fertile window starts 5 days before ovulation and ends on the ovulation day, 6 days in all.
- Later periods start one and two cycle lengths after the next period: (d + kc) + c and (d + kc) + 2c.
Cycle length can be 20 to 45 days and period length 1 to 10 days.
Assumptions
- Every cycle is as long as the one you enter. Real cycles often vary by a few days.
- Ovulation comes 14 days before the next period, the average in a 28-day cycle (ACOG). In real cycles this part, the luteal phase, varies too.
- The fertile window is the 6 days ending on ovulation day (Wilcox and others, 1995).
This page is for information only. It is not medical advice and not a method of birth control. See a health care professional if your periods are very irregular, very heavy, or stop.
Worked examples by hand
Last period September 1, 2026, 28-day cycle, 5-day period, today September 1 to 29. Next period: September 1 + 28 days = September 29, 2026. It ends 4 days later, on October 3. Ovulation: September 29 − 14 = September 15; the fertile window starts 5 days earlier, on September 10. The next two periods: September 1 + 56 days = October 27 and + 84 days = November 24.
Last period June 1, 2026, 28-day cycle, 5-day period, today September 28, 2026. 119 days have passed, and 119 ÷ 28 = 4.25, so k = 5. Next period: June 1 + 140 days = October 19, 2026, ending October 23. Ovulation is October 19 − 14 = October 5, and the fertile window starts September 30. The two after it: November 16 and December 14.
Last period August 31, 2026, 28-day cycle, today September 28, 2026. Exactly 28 days have passed, so k = 1 and the next period starts today, September 28. The one after it starts October 26.
Last period December 20, 2026, 32-day cycle, 4-day period, today December 20, 2026 to January 21, 2027. December has 31 days, so 11 days reach December 31 and 21 more reach January 21, 2027. It ends on January 24. Ovulation is January 7, and the fertile window starts January 2.
Last period February 10, 2028, 21-day cycle, 7-day period, today February 10 to March 2, 2028. 2028 is a leap year, so February has 29 days: 19 days reach February 29 and 2 more reach March 2, 2028. It ends on March 8. Ovulation is February 17; the fertile window starts February 12.
Other questions people ask
How do I calculate when my next period is due?
Add your cycle length to the first day of your last period. With a 28-day cycle and a last period that started on September 1, the next one should start 28 days later, on September 29. Add the cycle length again for each later period.
What is a normal cycle length?
The average is about 28 days, but few cycles are exactly 28. In a study of more than 600,000 cycles, the average was 29.3 days (Bull and others, 2019). ACOG advises teens to see a doctor if periods come more often than every 21 days or less often than every 45 days, or last more than 7 days.
How do I count my cycle length?
Count from the first day of bleeding of one period up to, but not including, the first day of the next. If your cycles vary, use the average of the last few.
When do I ovulate?
In an average 28-day cycle, ovulation happens about 14 days before the next period starts (ACOG). The calculator counts back 14 days from the predicted period. ACOG notes that the time from ovulation to the next period is the most consistent part of the cycle.
What is the fertile window?
The days when sex can lead to pregnancy. Wilcox and others (1995) found that nearly all pregnancies came from sex in the 6 days ending on the day of ovulation. ACOG gives the window as 5 days before ovulation to 1 day after.
Can I use this calculator for birth control?
No. It assumes every cycle is the same length and that ovulation is always 14 days before the period, and neither is true for everyone. Wilcox and others (2000) found that the fertile window is often earlier or later than expected, even in regular cycles.